This invention relates generally to tunable radio frequency resonant circuits and more particularly to self-protected magnetically tunable radio frequency resonant circuits.
As is known in the art, tunable radio frequency (r.f.) resonant circuits are often used in r.f. receivers to provide filtering, for example, of received r.f. energy. A particular class of tunable r.f. filters are those which can be magnetically tuned of which resonant circuits comprising a body of a ferrimagnetic material disposed between a pair of coupling circuits are the most common. An external d.c. magnetic field is applied to the body of the ferrimagnetic material and energy is fed to an input one of such coupling circuits. A portion of such energy is then coupled to the resonator body, if such portion of energy has a radian frequency (hereinafter frequency) (.omega..sub.i) which satisfies a resonance condition given as .omega..sub.i =.omega..sub.o, where .omega..sub.o, is a resonant frequency of the resonator, given as .omega..sub.o =.gamma.H.sub.DC, where .gamma. is a term referred to as the gyromagnetic ratio, and H.sub.DC is the magnitude of the applied D. C. magnetic field. In many prior art magnetically tunable resonators, the external magnetic field is supplied by a magnetic pole piece and a flux return yoke. The magnetically tuned resonant circuit is generally positioned in a gap between the flux return yoke and the magnetic pole piece.
As is also known in the art, it is often desirable to provide protection to the filter and the receiver, during transmission of energy such as in a radar system. This is particularly important when the receiver and filter are arranged such that transmitted r.f. energy from a high power transmitter leaks into a signal path of the receiver. Leakage of the high level energy into the receiver path either can damage components of the receiver or saturate the resonant body in the r.f. filter such that it is rendered temporarily inoperative. Thus, if during transmission the filter is tuned to the frequency of such transmitted energy, then the possibility exists wherein the filter will become saturated due to the high input power level. When the filter is saturated, a relatively long period of time may be required for the saturation effects to dissipate in order to make the filter and hence the receiver operative. During this time, the filter and hence the receiver cannot respond to an echo signal which would normally be received by the receiver and hence information contained in the echo signal will not be processed by the receiver. Several approaches have been used in the prior art to overcome this problem to provide protection to the resonant body in such filters and to the receiver. One approach is to include an r.f. limiter before the magnetically tuned resonant circuit to limit the r.f. signal which can pass therethrough. Aside from the cost of adding an additional component, this approach is also undesirable because the limiter has a finite insertion loss which reduces the sensitivity of the receiver to the echo signal. A second approach is to provide an electromagnet typically a wire coiled around a portion of the flux return yoke described above. The resonant frequency of the magnetically tuned resonator, is shifted by passing a current through the electromagnetic coil which creates a field either adding or opposing the d.c. magnetic field associated with magnetic pole piece, changing the resonant frequency of the filter in accordance with .omega..sub.o =.gamma.(H.sub.DC .+-.H.sub.DCP). One problem with the structure is that the electromagnetic coil due to its size, distance from the resonant body, high inductance and tight coupling to the core material has a frequency response unsuitable for rapidly changing the resonant frequency of a magnetically tuned resonator.
As is also known in the art, the resonance frequency of a YIG sphere is a function of temperature variations for most crystallographic orientations of the YIG sphere with respect to the external magnetic field H.sub.DC. However, along selective well-known orientations of the crystallographic axis of the sphere relative to the DC magnetic field, it is also well-known that the resonant frequency is substantially invariant with temperature variations. Generally, in the prior art, a partially orientated YIG sphere is disposed between the coupling loops and, in the presence of such loops, an iterative process is used where the resonant frequency of the filter is measured with the filter operating over the temperature range, and the sphere's orientation is established when the variation in resonant frequency is a minimum over the temperature range. This multi-step process is a time consuming process since two alignment steps are required. It has thus been a goal of YIG filter design to provide a YIG filter coupling structure having easy access to disposed therein a completely orientated YIG sphere having a proper orientation to minimize temperature variations in the resonant frequency of the output signal over the operating range of temperatures.